1. Field of the Invention
The present invention relates to an optical data storage medium such as optical disk on or from which information is optically written or read, by a laser beam, for example.
2. Discussion of the Prior Art
An example of a known magneto-optical data storage medium as an optical data storage medium is shown generally at 40 in FIG. 4. The data storage medium 40 includes a light-transparent substrate 42 which is formed of an acrylic or polycarbonate resin, glass or the like and has a tracking reflector 41, a protective layer 44 which is formed of SiO, AlN or similar material, a magneto-optical data storage layer 47 which is formed of GdTbFe, TbFeCo or similar material, another protective layer which is formed of AlN or similar material, and a reflecting layer 50 which is formed of Al or similar material. The protective layer 44, magneto-optical data storage layer 47, protective layer 48 and reflecting layer 50 are formed on the substrate 42 in the order of description. When information is written on this magneto-optical data storage medium 40, local spots on the magneto-optical storage layer 47 are irradiated with a laser beam and are thereby heated to a temperature near the Curie temperature. In this condition, each local spot is magnetized by exposure to an external magnetic field such that the direction of the magnetic field determines the direction of magnetization and thereby determines the logical value of the information to be written on the relevant local spot. The information written on each local spot on the magneto-optical data storage layer 47 is read based on the Kerr rotation angle of the polarization plane of a laser beam which is reflected from the portion of the medium 40 corresponding to the local spot. Namely, the amount of change in the Kerr rotation angle varies due to the Kerr effect, depending upon the direction of magnetization of the local spot. To increase the amount of change in the Kerr rotation angle due to the Kerr effect and thereby enhance the S/N ratio of a signal obtained from the reflected laser beam, the protective layer 44 and the reflecting layer 50 are provided to induce optical interference among these layers 44, 50 and the data storage layer 47. This increase in the Kerr rotation angle is referred to as "Kerr effect enhancement".
The tracking reflector 41 for tracking the recording track or tracks of the data storage layer 47 generally takes the form of a single spiral groove or a multiplicity of concentric grooves formed in the surface of the substrate 42 on the side of the data storage layer 47. In the presence of the tracking groove or grooves 41, the data storage layer 47, protective layers 44, 48 and reflecting layer 50 are locally recessed following the pattern of the groove structure.
To increase the Kerr effect enhancement by the optical interference among the protective, storage and reflecting layers 44, 47, 50, the thickness of the protective layer 44 is selected to be substantially equal to one quarter of the wavelength of the laser beam, and the thickness of the magneto-optical data storage layer 47 is selected so as to minimize the intensity of the laser beam which is directly reflected by the storage layer 47. This arrangement causes a relatively low intensity of the tracking laser beam which is reflected by the tracking groove or grooves 41, resulting in a low level of the tracking servo signal obtained from the tracking laser beam, and accordingly lowered reliability of the information reading laser beam due to misalignment of the beam with respect to the recording track. Further, the stepped or shoulder portions of the magneto-optical data storage layer 47 due to the tracking groove or grooves 41 tend to cause uneven or varying thickness at the edge portions of the recording track of the data storage layer 47, affecting the anisotropic properties (magnetizing direction) at the local spots on the recording track, and leading to instability of the anisotropic information to be read, i.e., low S/N ratio of the signal obtained from the information reading laser beam. Moreover, structural deterioration of the storage layer 47 and other layers may arise from the stepped portions 41, whereby the life expectancy of the data storage medium 40 is shortened.